Understanding factors influencing the aging process is central to reducing morbidity, an increasingly important goal in light of our rapidly aging population. Psychological stress is thought to accelerate biological aging. We found support for this hypothesis using the telomere length/telomerase maintenance system as an index of cell aging in vivo. Leukocyte telomere length (TL), the length of the DNA caps at the ends of chromosomes, shortens with age and predicts mortality. Telomerase is the cellular enzyme that lengthens and protects telomeres. Our preliminary data suggest that chronic psychological stress is associated with cell aging (PBMC TL shortening and decreases in telomerase activity) in two samples of caregivers, and further, prospective increases in stress are associated with decreases in TL over just one year. In vitro research has shown that TL is shortened most in specific cytolytic cells (CD8+CD28- T cells), especially from older donors, which is significant in that CD8+CD28- T cells secrete more proinflammatory cytokines and are a hallmark of immunosenescence. Thus, we will examine prospectively whether chronic stress precedes decreases in telomerase and TL in these cells, as well as in peripheral blood mononuclear cells (PBMCs), in older women. We will investigate putative biochemical mediators of this relationship, focusing on stress hormones and insulin resistance. In our pilot study, we also found that acute psychological stress, in contrast to chronic stress, is associated with acute increases in PBMC telomerase activity (telomerase reactivity). The magnitude of this increase was greater in caregivers than in controls. These are novel findings requiring replication with a larger sample. They raise the possibility that exaggerated initial telomerase responses to acute stress may signal unsuccessful attempts at telomere maintenance in chronically stressed individuals. To accomplish these aims, we will examine cell aging in a sample of 150 healthy postmenopausal women, aged 50-75, comparing chronically stressed caregivers (n = 75), to low stress controls (n = 75), matched on age, body fat, ethnicity, and education. We will test for stress-related differences in telomerase and TL in PBMCs, CD8 T cells (CD28+ and CD28-), CD4 T cells, and B cells, at baseline and 2 years later (Aim 1). Secondly, we will examine whether telomerase reactivity after acute lab stress predicts changes in TL over 2 years (Aim 2). Lastly, we will test in vitro whether T cells from high stress caregivers have impaired functional immune responses (greater proinflammatory cytokine response to stimulation, impaired cytotoxic function), and if these differences are mediated by lower basal telomerase and shorter telomeres (Aim 3). Together, these aims will provide a clearer picture of stress-induced immune senescence, elucidating the roles of telomeres and telomerase. Since shorter telomeres predict future disease and earlier mortality, understanding the risk factors for accelerated telomere shortening may facilitate interventions aimed at forestalling such outcomes.